1. Field of the Invention
The present invention relates to a planetary gear reduction mechanism for use in the starter of an internal combustion engine.
2. Description of the Related Art
FIG. 12 is a cross-sectional view of a known planetary gear reduction starter that is a motor having permanent magnets.
As shown, a DC motor armature 1 as a rotor is integrally attached on to an armature rotation shaft 2. A cylindrical yoke 3 surrounds the armature 1. The front end of the yoke 3 is received in a front bracket 5.
An inner gear 4 is integrally formed with the front end portion of the yoke 3. An inwardly projecting disk-like flange 4a is integrally formed with the front end of the yoke 3. The flange 4a has on its inner edge a cylindrical flange 4b extending toward the front. A sun gear 6 is integrally formed with the armature rotation shaft 2 on its front end.
A flange portion 10 that constitutes the arm of a planetary gear reduction mechanism is integrally formed with an output shaft 11. The output shaft 11 is supported by a sleeve bearing 12 that is received in and secured by the flange 4b. The flange portion 10 has a ring of through holes 10a that are circumferentially equally spaced in a circle. A support pin 9 is press-fit inserted into each of the through holes 10a. Each planetary gear 7 is supported on its bearing 8 that is attached to the corresponding pin 9. Each planetary gear 7 is meshed with the sun gear 6 and the inner gear 4 in a planetary gear reduction mechanism.
A sleeve bearing 13 is attached into the socket portion of the output shaft 11 open toward the rear side. The armature 2 is supported on its front end by the sleeve bearing 13. A steel ball 14 is disposed between the end of the armature rotation shaft 2 and the bottom of the socket of the output shaft 11, in order to receive thrust load.
Permanent magnets 15 whose number agrees with the number of the poles of the motor are arranged on the inner circumference of the yoke 3 surrounding the armature 1. A washer 16, disposed between the output shaft 11 and the flange 4a, restricts displacement of the output shaft 11 in the direction of thrust. The washer 16 also prevents the flange portion 10 from abutting the flange 4a of the inner gear 4 so that the output shaft 11 may rotate smoothly.
The operation of the above prior art planetary gear reduction starter is now discussed.
When a key switch (not shown) is closed, the armature 1 is power fed and activated. The armature 1 starts to rotate under the permanent magnets 15. The rotary motion of the armature 1 is transmitted to the planetary gear 7 via the sun gear 6 on the armature rotation axis 2. Since the planetary gear 7 is meshed with the inner gear 4, the planetary gear 7 rotates about its own shaft axis, namely about support pin 9, while revolving at the same time about the armature rotation 2. The rotary motion of the armature 1 is thus transmitted to the flange portion 10 with rotational speed reduced. The rotary motion of the armature 1 is further transmitted via the flange portion 10 to the output shaft 11. The rotary motion of the output shaft 11 is transmitted to a ring gear (not shown) of an internal combustion engine via the pinion (not shown) of an over-running clutch.
In the above prior art planetary gear reduction starter, the motor section is not isolated from its reduction section. The dust generated in the motor section is introduced into meshed portions of the planetary gears 7, the inner gear 4 and the sun gear 6, thereby damaging the planetary gear reduction mechanism.
To resolve the above problem, the motor section and the reduction section must be isolated by disposing a plate therebetween. However, the mounting of the plate is physically difficult because the yoke 3 and the inner gear 4 are integrally formed.